Very high frequency components in the power networks may result from events such as lightning surges and switching or fault transients. Such transients can destroy motors, transformers and other electrical equipment due internal resonant phenomena resulting in local amplification of voltage. Overstressing the insulation system reduces significantly the equipment lifetime and often leads to internal short-circuit. At very high frequencies wave propagation phenomena must be taken into account and thus wave impedance matching between particular components becomes important. The use of low-loss cables of wave impedance not matching the impedance of the equipment results in wave reflections generating repetitive HF overvoltages. A typical method of protecting the equipment against very fast transient (VFT) phenomena is the use of surge capacitors or RC-snubbers. The size and cost of installation of these devices are significant and thus they are rarely used in practice, especially for protecting MV equipment. In the present article a new approach towards suppressing VFTs with the use of compact series impedance element combined with a dedicated surge capacitor is presented. The approach developed enables one to construct a compact protective device introducing a series impedance already at significantly lower frequencies. The device is particularly suitable for protecting transformers connected to low loss cable lines with the use of a vacuum circuit breaker (VCB).
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